1. Field of the Invention
The invention relates to the metallurgical field and, more particularly, to a high carbon content and high strength heat-treated steel rail with excellent wear resistance and plasticity as well as a method for producing the same.
2. Description of the Related Art
One of the effective methods for prolonging the service life of steel rails is to improve the strength thereof. Pearlite, tempered martensite, and bainite are common structures for producing steel rails, among which pearlite structures are widely used due to good wear resistance, a simple production process, their low cost, and stable properties. However, for a pure pearlitic steel rail, the strength thereof hardly exceeds 1,330 MPa and the surface hardness hardly exceeds 380 HB. That is to say, the rail strength has very limited room for improvement.
Carbon is an effective element for improving the wear resistance of steel rails. Improvements in the cementite content of lamellar pearlite can improve the wear resistance. It is well known in metallurgic sciences that when the carbon content of a steel exceeds 0.77%, a proeutectoid cementite (secondary cementite) first forms under equilibrium. However, if the cooling rate is accelerated during the transformation of steel from austenite structures to pearlite structures, even if the carbon content exceeds 0.77%, a pseudo-eutectoid pearlite forms rather than the proeutectoid cementite. With the acceleration of the cooling rate, the upper limit of the carbon content of the pseudo-eutectoid pearlite increases. In use, railheads generally wear to a depth of 20 mm. To ensure safe use of the rails, the carbon content of the steel rails must be enhanced so that the pearlite structures are distributed to a depth of at least 25 mm from the railhead surface.
Conventional methods for producing high strength heat-treated steel rails employ eutectoid steel with a carbon content of 0.60-0.82%. The high strength is achieved by generating fine pearlite structures. However, if the rails have a low carbon content, the density of the cementite structures in the steel is low, and the tensile strength is low, generally less than 1,330 MPa. Thus, the rails have a poor wear resistance and short service life.
In the prior art, methods for producing steel rails with good wear resistance make use of hypereutectoid steel with a carbon content of 0.85-1.40%. The good wear resistance is achieved by generating fine pearlite structures and increasing the cementite density in the pearlite lamella. However, the methods have the following disadvantages. First, the obtained steel rails still have a low strength, generally less than HB 380, and the tensile strength is generally less than 1,330 MPa. Second, because the pearlite structures are distributed to a depth of only 20 mm from the surface, phase segregation occurs. The proeutectoid cementite structures, therefore, precipitate, which deteriorates the rail properties and provides a source for fatigue cracks and brittle fractures. Third, conventional cooling rates are generally less than 10° C./s, usually 2-5° C./s. To improve the rail strength, a high cooling rate (5-15° C./s) is required, which would require the existing production lines to be updated, incurring a high investment. Finally, nitrogen is harmful for rail properties, but conventional methods have no way of reducing this harm.
As the carbon content increases, the plasticity and toughness of the rails decrease. Thus, compared with common pure pearlite structures, the hypereutectoid rails have a much lower plasticity and toughness, which means the rails may break when use in cold regions with temperatures below zero. Although the prior art discloses that plasticity and toughness may be enhanced by cooling different portions of the rails with different modes, the operation is complicated and has a high cost.
Thus, it is urgent to develop a high carbon content and high strength hot rolling steel rail with good wear resistance and plasticity and a method for producing the same.